Variable impedance transformer



g- 59 R. w. ALLEN ET AL 2,900,610

VARIABLE IMPEDANCE TRANSFORMER Filed May 19, 1955 L/A/E 12 INVENTORS R/cwA/w M4 AALEA/mva By /r'fl Come/v 7Z/A/m/6 2,900,610 Patented Aug. 18, 1959 2,900,610 VARIABLE nvnnnnncn TRANSFORMER Richard W. Allen, Haddon Heights, N.J., and Fred Cohen, Philadelphia, Pa., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army Application May 19,1955, Serial No. 509,486

1 Claim. (Cl. 333-35) This invention relates to a variable impedance transformer, and more particularly to an output transformer for use at ultra high frequencies for coupling an output from a tunable resonant cavity to a utilization circuit at any desired operating frequency over a broad frequency range.

This invention is particularly useful in coupling an output from a resonant cavity defined by inner and outer coaxial conductors which are coupled at one end to output electrodes of a vacuum tube and which have an annular tuning plunger at the other end. A resonator and output transformer capable of handling relatively high power, such as kilowatts or more, should include elements arranged with sufficient clearance to avoid arcing and should employ sliding electrical contacts only where the voltage and current conditions are low enough to avoid deterioration of the sliding contacts. It is therefore desirable to permanently connect the inner conductor of an output line to the inner conductor of the resonator, and the outer conductor of the line to the outer conductor of the resonator. However, the point in the resonator to which the line is connected presents grossly different impedances at different frequencies to which the resonator is tuned by means of the tuning plunger. It is therefore a general object of this invention to provide an improved variable output transformer between a resonator and a coaxial output transmission line whereby the impedance presented by the resonator may be easily transformed to the impedance of the output transmission line at any frequency of the resonator over a broad frequency range.

It is another object to provide an improved vacuum tube output circuit and output transformer suitable for handling high power at ultra-high frequencies.

It is a further object to provide a simple and easily constructed novel form of variable output transformer.

One embodiment illustrative of the invention comprises a quarter-wave resonant cavity of coaxial conductors coupled at one end to output electrodes of a vacuum tube and adjustably terminated at its effective other end by an annular tuning plunger. A first coaxial line section of relatively high characteristic or surge impedance is permanently connected at one end to the cavity conductors at a point electrically near the tuning plunger (the low voltage end of the cavity). The other end of the first coaxial line section is connected to a coaxial output transmission line of lower characteristic impedance. An axially moveable slug is positioned in the output line. The first coaxial line section and the slug have a length equal to a quarter-wavelength at a frequency in the middle of'the tuning range of the resonant cavity. The slug is moveable axially to provide a second coaxial line section, between the first coaxial line section and the slug, which is variable in length from zero to a quarter-wavelength. The resonant cavity may be tuned to any frequency over a broad range, such as from 325 to 425 megacycles. The impedance presented by the resonant cavity at the output connection may then vary from 400 ohms at the low frequency to 25 ohms at the high he quency. The slug can be adjusted to transform the output impedance of the resonant cavity to the characteristic impedance of the output transmission line, which may be 50 ohms, or to an impedance providing a match when the output line is connected to a load such that there is a voltage standing wave ratio of up to about 1.5 in the line due to the load. A relatively small amount of reactive admittance added by the transformer in parallel with the resonant cavity is neutralized by a slight readjustment of the cavity tuning plunger.

These and other objects and aspects of the invention will be apparent to those skilled in the art from the following more detailed description taken in conjunction with the single appended drawing showing a vacuum tube coupling constructed according to the teachings of this invention.

A vacuum tube 5 includes a cathode electrode metallic contact ring 6 and an anode electrode metallic contact ring 7. The vacuum tube 5 may be an RCA type 6448 vacuum tube useful for providing high power at ultrahigh frequencies. A tunable coaxial resonant output cavity 8 is defined by an inner conductor 9, an outer conductor 10 and an annular tuning plunger 11. The tuning plunger includes resilient metallic contact fingers engaging the adjacent surfaces of conductors 9 and 10 as shown. The lower ends of the coaxial conductors 9 and 10 are coupled or connected to the anode contact ring 7 and the cathode contact ring 6, respectively, of the vacuum tube 5. The resonant cavity 8 has an axial length equal electrically to a quarter-wavelength at the resonant frequency. A type 6448 tube is characterized in having internal physical dimensions such that, in effect, an appreciable length of transmission line exists inside the tube. Therefore, a large portion of the quarterwavelength cavity 8 is electrically inside the vacuum tube. It will, of course, be understood that whenever reference is made to a quarter-wavelength section, that the section may include an odd multiple of quarter-wavelengths.

An output coaxial transmission line 12 for transmitting high'frequency energy away from the vacuum tube to a suitable load or utilization device includes an inner conductor 13 and an outer conductor 14. The output line 12 may have a characteristic impedance of 50 ohms. It is desired to couple the output line 12 to the resonant cavity 8 in such a way that an impedance match can be achieved at any desired operating frequency as determined by the position of the output tuning plunger 11. intervening coaxial line sections 15, 16 and 17 are provided for this purpose.

The coaxial line section 15 includes an inner conductor 20 connected at one end to the inner conductor 9 of the resonant cavity, and an outer conductor 21 connected to the outer conductor 10 of the resonant cavity 8. The conductors of the line section 15 are permanently connected, as by welding, to the corresponding conductors of the cavity 8. By this construction the connection is capable of handling high radio frequency power and is free of the limitations imposed by coupling loops, probes and sliding contacts. The coaxial line section 15 has an axial length of a quarter-wavelength at a frequency in the middle of the tuning range of the resonant cavity 8. The coaxial line section 15 may, for example, be dimensioned to have a characteristic impedance of ohms. I v

The other end of the coaxial line section 15 is connected to the respective inner and outer conductors of the coaxial line section 16 which preferably has the same characteristic impedance as the output transmission line 12. The coaxial line section 16 is, in turn, connected to the respective conductors-of a coaxial line section '17 having a lower characteristic impedance than the output line 12. The coaxial line section 17 includes an axially moveable slug 22 having an electrical length equal to a quarter-wavelength at a frequency in the middle of the tuning range of the resonant cavity 8. The slug 22 is axially moveable by means of a handle 23 extending thru a slot 24 in the outer conductor, or by any other. suitable mechanical means. The characteristic impedance of the coaxial line section 17 including the slug 22 is relatively low, such as 20 ohms. The slug 22 is illustrated in the drawing as an electrically conductive cylinder, the ends of which contact the walls of the outer conductor 14. The ends of cylindrical slug 22 are provided with resilient metallic contact fingers. It will be understood that the slug 22 may be a conductive cylinder contacting the .walls of the inner conductor 13, or may consist of a cylindrical dielectric member having dimensions and dielectric constant to provide the desired characteristic impedance. For example, the dielectric material may be tetrafluoroethylene, known as Teflon, which has a dielectric constant of about two.

The characteristic impedances of the coaxial line sections 15, 16 and 17 are progressively lower in value, and may for example have values of 80, 50 and 20 ohms, respectively. The characteristic impedance of a coaxial transmission line varies as a logarithmic function of the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor. Therefore, with an inner conductor of constant diameter, the characteristic impedance will vary as a direct logarithmic function of the diameter of the outer conductor. This is illustrated in the drawing. It is, of course, possible to achieve the same result by varying the diameter of the inner conductor only, or by varying the diameters of both the inner and outer conductors, or by varying the dielectric material between the conductors. Any desired characteristic impedance is obtained according to the formula:

where k is the dielectric constant, D is the diameter of the outer conductor, and d is the diameter of the inner conductor.

The coaxial line sections 16 of intermediate characteristic impedance is adjustable in length by moving the slug 22 forming part of section 17. The length of the coaxial line section 16 is variable over a quarter-wavelength range such as from zero length to a length equal to a quarter-wavelength at a frequency in the middle of the tuning range of the resonant cavity 13. Alternatively,

it may be convenient to make section 16 variable from a quarter-wavelength to a half-Wavelength.

The point at which the coaxial line section is connected to the resonant cavity 8 is a point where a relatively low radio frequency voltage exists between the inner and outer conductors of thecavity 8. The voltage difference between the conductors is zero at the tuning plunger 8 and is a maximum electrically a quarter-wave distant inside the vacuum tube 5. The major portion of the electrical quarter-wavelength is within the tube 5. Therefore, the point at which the coaxial line section 15 is connected is electrically relatively near the tuning plunger 11, and is a point of relatively low'radio frequency voltage. v

In operation, the tuning plunger 11 is adjusted to provide a desired resonant frequency in the cavity 8. The slug 22 is then adjusted in axial position to provide an impedance transformation which permits the maximum transfer of energy from the resonant cavity 8 to the load connected to the output transmission line 12. The impedance transformer constituted'by the coaxial line sections 15, 16 and 17 introduces a small reactive component of admittance into the resonant cavity 8. This reactive component introduced is relatively small by comparison with the reactive component of admittance at that point in the cavity itself. It is therefore possible to neutralize the reactive component introduced by the transformer by a slight readjustment of the position of the tuning plunger 11 in the resonant cavity 8. The impedance transformer provides a match even though the output line 12 is connected to a load causing a voltage standing wave ratio of up to about 1.5.

In an output coupling system constructed according to the invention and successfully operated, the resonant cavity 8 was tunable over a range of from 325 to 425 megacycles. The impedance presented by the resonant cavity at the point of connection of the coaxial line section 15 varied from 400 ohms at the low frequency to 25 ohms at the high frequency. The coaxial line sections 15, 16 and 17 had values of characteristic impedance equal to 80, 50 and 20 ohms, respectively. It was found that the slug 22 could be adjusted to match the output line 12 to the cavity 8 at any desired operating frequency in the range of from 325 megacycles to 425 megacycles.

The selection of characteristic impedances for the coaxial line sections is preferably made by plotting the impedance transformations on a Smith chart. The greater the tuning range over which the resonant cavity 8 is to be operated, the greater the difference necessary in the characteristic impedance of the coaxial line sections 15 and 17. Values of characteristic impedance can be determined which provide the necessary impedance transformation while introducing a minimum of reactive admittance in parallel with the resonant cavity 8.

It is apparent that according to the teachings of this invention there is provided a simple and easily constructed transformer and resonant circuit arrangement whereby the output of the cavity can be matched to a lead over a broad range of operating frequencies, the arrangement being capable of handling high radio frequency power at ultra-high frequencies.

What is claimed is:

A coupling for a vacuum tube comprising, a tunable resonant cavity defined by inner and outer coaxial conductors adapted at one end for coupling to electrode contact rings of a vacuum tube and having a tuning plunger at the other end, a coaxial line having inner and outer conductors, a coaxial line section having inner and outer conductors connected at one end to the respective inner and outer conductors of said cavity and connected at the other end to the respective inner and outer conductors of said coaxial line, and a slug positioned between said inner and outer conductors of said coaxial line, said slug being longitudinally moveable within said coaxial line and arranged so that it contacts said outer conductor of said coaxial line throughout its movement, said line section and said slug having a length equal to a quarter-wavelength at a frequency in the tun ing range of said cavity, said line section having a characteristic impedance greater than the characteristic impedance of said line.

References Cited in the file of this patent UNITED STATES PATENTS 1,921,117 Darbord Aug. 8, 1933 1,927,393 Darbord Sept. 19, 1933 2,125,597 White et a1. Aug. 2, 1 938 2,403,252 Wheeler July 2, 1946 2,514,344 Slaymaker et al. July 4, 1950 2,643,296 Hansen June 23, 1953 2,771,516 Bucksbaum Nov. 20, 1956 2,790,857 Gluyas et a1. Apr. 30, 1957 FOREIGN PATENTS 572,739 Great Britain Oct. 22, 1945 

